• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
From what I can tell, it simply gives you more of the resource in question without a traceable source.

In other words, if the sum of all of your individual provinces metal production is, say, 120, you would get the 120 plus the additional "off map" amount. Sort of a simplified way of granting a constant bonus.

I could be wrong, but from my observations based on inductive logic, I think I am probably correct.